I'm not sure what you mean by a "spaceflight feature". I suspect Celestia does not have it, though. You might want to investigate Orbiter, which really does simulate spaceflight, from launch into orbit, through interplanetrary trajectories with mid-flight corrections, to orbiting around and landing on remote planets.
See
http://www.medphys.ucl.ac.uk/~martins/orbit/orbit.html
Celestia has a few controls that make the viewpoint travel at specific speeds through space. You also can use a joystick to control the viewpoint, although it seems to have problems with some types of joysticks. The Controls entry on the Help menu lists some of the commands. You might also consider reading the Celestia Users Manual, which shows how to use Celestia. It's on the CelestiaMotherlode at
http://www.celestiamotherlode.net/catal ... ation.html
Addons for Celestia define objects in fixed orbits that you can go look at. If you added the Atlantis spacecraft, you can now use a GoTo command to go see it. You can't use Celestia to fly it around.
(The next release of Celestia v1.5.0 makes it possible to change the orientation of objects while Celestia is running. It
might be able to change their orbits, but that isn't clear, yet. "Possible" does not mean "easy".)